Conventionally, a heat exchanger which works as an evaporator or a condenser is employed in air-conditioning equipment such as a home air conditioner, a vehicle air conditioner or a package air conditioner, a refrigerator or the like. In the home air conditioner for indoor use and the package air conditioner for business use, a cross fin tube type heat exchanger is the most generally used. The cross fin tube type heat exchanger is constructed such that aluminum plate fins on an air side and heat transfer tubes (copper tubes) on a refrigerant side are fixed integrally to each other. As the heat transfer tube for such a cross fin tube type heat exchanger, there is well known a so-called internally grooved heat transfer tube which includes a multiplicity of spiral grooves formed on its inner surface so as to extend with a prescribed lead angle with respect to an axis of the tube and internal fins having a predetermined height and each formed between adjacent two of the grooves.
In such an internally grooved heat transfer tube, for attaining high performance of the heat exchanger, the internal grooves are made deeper and the internal fins formed between the grooves are made narrower. Further, there have been proposed various heat transfer tubes which purse high performance by optimizing the groove depth, an apex angle of the internal fins, the lead angle, a cross sectional area of the grooves and so on.
As a refrigerant used in this kind of cross fin tube type heat exchanger, there have been conventionally used fluorocarbon refrigerants (Freon refrigerants) such as R-12, R-22 and the like in view of the danger of catching fire and exploding at the time of leakage thereof and the efficiency of the heat exchanger. However, as the global environmental problems become serious in these years, CFC and HCFC refrigerants containing chlorine are being replaced with HFC refrigerants from the standpoint of prevention of destruction of the ozone layer. Further, among those HFC refrigerants, R-407C and R-410A having relatively high global warming potential are being positively replaced, from the standpoint of prevention of global warming, with other HFC refrigerants such as R-32 having low global warming potential and natural refrigerants such as a carbon dioxide gas, propane and isobutene. In particular, because the carbon dioxide gas refrigerant has no toxicity to human bodies and non-flammability, unlike other natural refrigerants such as propane, the danger of catching fire or the like due to its leakage is low. Accordingly, the carbon dioxide gas has been attracting attention as a refrigerant used in an air-conditioning refrigerating water supply system having an air-conditioning function and a refrigerating or freezing function.
Where such a carbon dioxide gas (CO2) is used as the refrigerant for the refrigerating air-conditioning water supply apparatus, however, a supercritical cycle is applied in which a pressure region above a critical point of the refrigerant is utilized on a high-pressure side, unlike a refrigerating cycle of a heat exchanger using ordinary HFC refrigerants and so on. The pressure on the high-pressure side varies depending upon use or application of the heat exchanger (freezing, air conditioning, water supply). In considering a maximum operating pressure of the heat exchanger, reliability evaluating conditions of a compressor for the water supply system is referred to. For instance, in a long-time reliability test for evaluating the reliability of the compressor for the water supply system, the operating pressure of about 15 MPa is employed. While there is data that a coefficient of performance (COP) of such a water supply system becomes maximum around 12 MPa, it is preferable to design the heat exchanger so as to have pressure resistance at its operating pressure of about 15 MPa at maximum, in consideration of unexpected changes in operating conditions. Namely, in a case where the conventional refrigerants are used, the heat exchanger is operated at a pressure of about 1-4 MPa. In contrast, where the carbon dioxide gas refrigerant is used, the heat exchanger is operated at a high pressure of 5-15 MPa, which is about five times higher than that in the conventional case.
Thus, in the cross fin tube type heat exchanger using the carbon dioxide gas refrigerant, since the heat transfer tube (the internally grooved heat transfer tube) through which the refrigerant flows tends to suffer from a considerably high pressure, it is required to enhance the strength for pressure resistance of the heat transfer tube. For this end, there are employed various techniques such as a reduction in the diameter of the heat transfer tube, a change in the material for the tube, an increase in the groove bottom thickness, etc. As the techniques of the reduction in the diameter of the heat transfer tube and the change in the material for the tube, JP-A-2002-31488 (Patent Publication 1) discloses, for instance, use of small-diameter copper or stainless tubes. In JP-A-2001-153571 (Patent Publication 2), for instance, a heat exchanger is formed by flat, elliptical aluminum tubes with a multiple holes. However, the change in the material for the heat transfer tube to stainless or aluminum undesirably may result in deteriorated workability of the tube or poor bonding of the tube. Accordingly, it is preferable that the material for the heat transfer tube be copper or a copper alloy. In the above-indicated Patent Publication 1, the small-diameter copper-made heat transfer tube is disclosed. The disclosed heat transfer tube, however, has a smooth inner surface and accordingly its heat transfer performance is insufficient as compared with the internally grooved heat transfer tube. Therefore, from the viewpoint of improvement in the heat transfer performance, it is desired to provide the internally grooved heat transfer tube having a high degree of strength for pressure resistance and made of the copper or copper alloy.
In the internally grooved heat transfer tube made of the copper, there are employed, for enhancing the strength for pressure resistance, various techniques such as the reduction in the outside diameter of the tube and the increase in the groove bottom thickness which is a thickness of the tube at a portion thereof corresponding to each groove formed on its inner surface. As for the reduction in the diameter of the tube, it is possible to reduce the diameter from about 7 mm that is a generally employed value to about 4 mm. In a heat exchanger of an air cooling type, the heat transfer tube is fixed to heat-dissipating fins usually according to a mechanical tube-expanding method in which a tube-expanding plug is inserted through the heat transfer tube for expanding the tube, whereby the heat transfer tube is brought into close contact with and fixed to the heat-dissipating fins in mounting holes formed in the fins. Therefore, it is technically difficult to fix the heat transfer tube with the diameter of 6 mm or smaller to the heat-dissipating fins by the mechanical tube-expanding method. In the meantime, in a case where the strength for pressure resistance is enhanced by increasing the groove bottom thickness, a large force is required in the mechanical tube-expanding operation for expanding the tube wall with increased groove bottom thickness by the tube-expanding plug inserted in the tube. Accordingly, it is rather difficult to employ the mechanical tube-expanding method unless the heat transfer tube with a relatively large diameter is used. As another method for expanding the tube, there is known a hydraulic tube-expanding method in which a liquid is charged into a fluid-tightly sealed heat transfer tube and a pressure is applied to the charged fluid, thereby expanding the tube. This hydraulic tube-expanding method requires a complicated arrangement and is inferior in view of mass production.
Further, in the current technique of manufacturing the internally grooved heat transfer tube, since the groove depth tends to be decreased with an increase in the groove bottom thickness, it is difficult to improve the heat transfer performance of the internally grooved heat transfer tube by employing techniques for attaining high performance such as an increase in the height of the internal fins and a decrease in the width of the internal fins. In addition, in the case where the groove bottom thickness is increased, a large force acts on the tube when the tube is expanded by the mechanical tube-expanding method, causing a problem that the fins are collapsed due to the pressure upon the mechanical tube expanding if the fins each formed between adjacent two grooves on the inner surface of the tube are configured to have an increased height or an increased width.
In the light of the foregoing, it is not preferable from the viewpoint of the design for pressure resistance to employ the conventional internally grooved heat transfer tube whose performance has been enhanced by the increase in the height of the fins or the decrease in the width of the fins, as the internally grooved heat transfer tube used for the heat exchanger of the refrigerating air-conditioning water supply apparatus using the refrigerant whose pressure is higher than that of the conventionally used refrigerant. Further, it is not desirable to change the material for the heat transfer tube and reduce the outside diameter of the tube in an attempt to improve the strength for pressure resistance since the change in the material and the reduction in the tube diameter lead to deteriorated workability. Moreover, where the strength for pressure resistance is enhanced simply by increasing the groove bottom thickness, the groove depth is reduced due to limitation in working under the present circumstances. Therefore, it is indispensable to develop a groove structure which assures high heat transfer performance, on the premise that the groove depth is made smaller than before.
Patent Publication 1: JP-A-2002-31488
Patent Publication 2: JP-A-2001-153571